In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each cellular telephone. To place a telephone call, the individual subscriber or other authorized user of a cellular telephone dials a telephone number and presses the "Send" button. In response, the cellular telephone transmits its ESN and MIN to the cellular system so the individual subscriber can be charged for the telephone call.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by "cloning" the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular system without paying for the service. A pirate can obtain the ESN and MIN of a cellular telephone by monitoring a transmission of the cellular telephone, and then programming them into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN is inadequate to protect a cellular telephone system from illegal use by pirates.
In an effort to provide additional security, some cellular systems authenticate cellular telephones based on the transmission of data by the cellular telephone during a call set-up process. Rather than identify the cellular telephone by its ESN and MIN alone, the system identifies a cellular telephone by its transmission characteristics. In this manner, the cellular system can reject calls from cloned cellular telephones even when those cellular telephones transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), incorporated herein by reference, a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient reference waveform is used to identify the type of transmitter. The transmission characteristics of the reference waveform can be processed in different manners to create a "fingerprint" of the individual transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other fingerprint characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910, issued to Rudokas on May 30, 1995 ("the Rudokas patent"), incorporated herein by reference, describes an identifier, such as a radio frequency signature, that can be used to positively identify a valid cellular telephone or a known fraudulent telephone. Other types of signature or fingerprint authentication systems are also known in the art.
Fingerprint authentication systems all require at least one transmission characteristic waveform, known to be generated by the authentic cellular telephone, to be used as a reference waveform for the fingerprint authentication system. Some systems may rely on more than one reference waveforms to generate the fingerprint. The term fingerprint as used herein is intended to include reference waveforms transmitted by a transmitter as well as the various transmission characteristics derived from those reference waveforms.
An effective fingerprint identification technique depends largely on the existence and quality of a database of stored fingerprints. In the Ferrell patent, a database of valid reference waveforms are collected by waiting for the transmitters to turn on. Not surprisingly, this type of a passive collection process may take a very long time to develop due to the unpredictability of the turn-on behavior of the transmitters. Moreover, such a passive collection process is impractical in a communications network such as a cellular telephone system in which there are millions of cellular telephones whose reference waveforms need to be collected.
Moreover, even when a database of high quality fingerprints exists, there are times when the stored fingerprint of a particular authorized cellular telephone may not reflect the true transmission characteristics of that cellular telephone. One reason is that the transmission characteristics of the authorized cellular telephone may have changed over time due to the aging or peculiar nature of the transmitter circuits in the authorized cellular telephone. Another reason is that a sophisticated pirate may have discovered how the fingerprint authentication system collects the reference waveforms and corrupted the stored fingerprint with a fingerprint of the unauthorized cellular telephone. As can be appreciated, this causes a serious problem for both the valid subscriber using the authorized telephone and the cellular service provider.
If the fingerprint authentication system misidentifies a valid call from the authorized telephone as fraudulent and blocks the call, the result is serious inconvenience to the valid subscriber and loss of revenue for the cellular service provider. On the other hand, if the fingerprint authentication system identifies a call from the pirate's unauthorized cellular telephone as authorized and allows the calls, it results in loss of revenue for the cellular service provider. Both situations are to be avoided if possible.
Therefore, it can be appreciated that there is a significant need for a system and method for rapidly collecting reference waveforms from an authorized cellular telephone to establish or reestablish a valid fingerprint. There is also a significant need to ensure that the reference waveforms being collected are transmitted from the authorized cellular telephone rather than from a pirate's unauthorized cellular telephone. The present invention provides these and other advantages as will be illustrated by the following description and accompanying figures.